Driving method of liquid crystal display device

ABSTRACT

It is an object to provide a specific driving method for reduction in power consumption in displaying a 3D image with field sequential driving. A driving method of a liquid crystal display device is a method in which a stereoscopic image can be perceived with a liquid crystal display device switching an image for a left eye and an image for a right eye to display the image for the left eye or the image for the right eye, and a pair of glasses having a switching means with which the image for the right eye and the image for the left eye are switched in synchronization with display of the image for the left eye or the image for the right eye in order that the left or right eye of a viewer may selectively perceive the image for the left eye or the image for the right eye; the image for the left eye and the image for the right eye are perceived by the left eye or right eye in a mixed color by switching light which is emitted from a backlight portion and which corresponds to a plurality of colors, within a predetermined period, and the light which is emitted from the backlight portion are continuously emitted in accordance with an image signal of each of a plurality of colors which forms the linage for the left eye and the image for the right eye.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/072,864, filed Mar. 28, 2011, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2010-080794 on Mar. 31, 2010, both of which are incorporated byreference.

TECHNICAL FIELD

The present invention relates to a method for driving a liquid crystaldisplay device. Further the present invention relates to a liquidcrystal display device. Further the present invention relates to anelectronic device including the liquid crystal display device.

BACKGROUND ART

Liquid crystal display devices ranging from a large display device suchas a television receiver to a small display device such as a mobilephone have been spreading. From now on, products with higher addedvalues will be needed and are being developed. In recent years, liquidcrystal display devices with which viewers can experience virtualstereoscopic view have been developed in order to display a morerealistic image.

In addition, there has been a growing interest in global environment andthe development of liquid crystal display devices consuming less powerhas thus attracted attention. Thus, a driving method called a fieldsequential driving method (hereinafter, field sequential driving) hasbeen developed. In field sequential driving, backlights of red(hereinafter, sometimes abbreviated to R), green (hereinafter, sometimesabbreviated to G), and blue (hereinafter, sometimes abbreviated to B)are switched within a predetermined period, and light of R, G, and B aresupplied to a display panel. Therefore, a color filter is notnecessarily provided for each pixel, and use efficiency of transmittinglight from a backlight can be enhanced. Further, because one pixel canexpress R, G, and B, it is advantageous that improvement in definitionis easily realized.

Patent Document 1 discloses a structure in which a left image for a lefteye and a right image for a right eye are alternately displayed at highspeed with field sequential driving so that a viewer virtually perceivesthe images as a stereoscopic image.

REFERENCE

[Patent Document 1]Japanese Published Patent Application No. 2003-259395

DISCLOSURE OF INVENTION

As described in Patent Document 1, it is necessary to frequently repeatturning on and off a light source of a backlight in order that a 3D(three-dimensional or stereoscopic) image be displayed with fieldsequential driving. In the case of turning on the backlight in inputtingthe image signal, such driving causes a problem in that display isperceived before writing an image signal to each pixel has beenfinished. Therefore, in order to avoid the display problem, thebacklight is necessarily turned off whenever an image signal is written.However, with the structure in which the light is switched, which isemitted from the backlight, and which corresponds to respective colors,and the light are repeatedly turned on/off, a driver circuit operates bywhich a light source corresponding to colors is driven and switched;accordingly, power consumption is increased.

It is an object of an embodiment of the present invention to provide aspecific driving method for lower power consumption in displaying a 3Dimage with field sequential driving.

An embodiment of the present invention is a driving method of a liquidcrystal display device with which a stereoscopic image is perceived witha liquid crystal display device configured to switch an image for a lefteye and an image for a right eye to display the image for the left eyeor the image for the right eye, and a pair of glasses having a switchingmeans with which the image for the left eye and the image for the righteye are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer selectively perceives the image for the left eye or theimage for the right eye. The image for the left eye and the image forthe right eye are perceived by the left eye or right eye in a mixedcolor by switching light which is emitted from a backlight portion, andwhich corresponds to a plurality of colors, within a predeterminedperiod. The light which is emitted from the backlight portion arecontinuously emitted during a lighting period for the left eye or alighting period for the right eye which corresponds to one of theplurality of colors.

An embodiment of the present invention is a driving method of a liquidcrystal display device with which a stereoscopic image is perceived witha liquid crystal display device configured to switch an image for a lefteye and an image for a right eye to display the image for the left eyeor the image for the right eye, and a pair of glasses having a switchingmeans with which the image for the left eye and the image for the righteye are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer selectively perceives the image for the left eye or theimage for the right eye. The image for the left eye and the image forthe right eye are perceived by the left eye or right eye in a mixedcolor by switching light which is emitted from a backlight portion, andwhich corresponds to a plurality of colors, within a predeterminedperiod. The light which is emitted from the backlight portion arecontinuously emitted during a lighting period for the left eye or alighting period for the right eye which corresponds to one of theplurality of colors. When the image for the left eye and the image forthe right eye are switched, the light which is emitted from thebacklight portion toward the left eye and the right eye are blocked withthe pair of glasses having the switching means.

An embodiment of the present invention is a driving method of a liquidcrystal display device with which a stereoscopic image is perceived witha liquid crystal display device configured to switch an image for a lefteye and an image for a right eye to display the image for the left eyeor the image for the right eye, and a pair of glasses having a switchingmeans with which the image for the left eye and the image for the righteye are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer selectively perceives the image for the left eye or theimage for the right eye. The image for the left eye and the image forthe right eye are perceived by the left eye or right eye in a mixedcolor by switching light which is emitted from a backlight portion, andwhich corresponds to colors of red, green, and blue, within apredetermined period. The light which is emitted from the backlightportion are continuously emitted during a lighting period for the lefteye or a lighting period for the right eye which corresponds to one ofthe colors of the red, the green, and the blue.

An embodiment of the present invention is a driving method of a liquidcrystal display device with which a stereoscopic image is perceived witha liquid crystal display device configured to switch an image for a lefteye and an image for a right eye to display the image for the left eyeor the image for the right eye, and a pair of glasses having a switchingmeans with which the image for the left eye and the image for the righteye are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer selectively perceives the image for the left eye or theimage for the right eye. The image for the left eye and the image forthe right eye are perceived by the left eye or right eye in a mixedcolor by switching light which is emitted from a backlight portion, andwhich corresponds to red, green, and blue, within a predeterminedperiod. The light which is emitted from the backlight portion arecontinuously emitted during a lighting period for the left eye or alighting period for the right eye which corresponds to the red, thegreen, and the blue. When the image for the left eye and the image forthe right eye are switched, the light which is emitted from thebacklight portion toward the left eye and the right eye are blocked withthe pair of glasses having the switching means.

An embodiment of the present invention is a driving method of a liquidcrystal display device with which a stereoscopic image is perceived witha liquid crystal display device configured to switch an image for a lefteye and an image for a right eye to display the image for the left eyeor the image for the right eye, and a pair of glasses having a switchingmeans with which the image for the left eye and the image for the righteye are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer selectively perceives the image for the left eye or theimage for the right eye The image for the left eye and the image for theright eye are perceived by the left eye or right eye in a mixed color byswitching light which is emitted from a backlight portion, and whichcorresponds to colors of red, green, blue, and white, within apredetermined period. The light which is emitted from the backlightportion are continuously emitted during a lighting period for the lefteye or a lighting period for the right eye which corresponds to one ofthe colors of the red, the green, the blue, and the white.

An embodiment of the present invention is a driving method of a liquidcrystal display device with which a stereoscopic image is perceived witha liquid crystal display device configured to switch an image for a lefteye and an image for a right eye to display the image for the left eyeor the image for the right eye, and a pair of glasses having a switchingmeans with which the image for the left eye and the image for the righteye are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer selectively perceives the image for the left eye or theimage for the right eye. The image for the left eye and the image forthe right eye are perceived by the left eye or right eye in a mixedcolor by switching light which is emitted from a backlight portion, andwhich corresponds to red, green, blue, and white within a predeterminedperiod. The light which is emitted from the backlight portion arecontinuously emitted during a lighting period for the left eye or alighting period for the right eye which corresponds to the red, thegreen, the blue, and the white. When the image for the left eye and theimage for the right eye are switched, the light which is emitted fromthe backlight portion toward the left eye and the right eye are blockedwith the pair of glasses having the switching means.

In the driving method of the liquid crystal display device according toan embodiment of the present invention, the light which may be emittedfrom the backlight portion have colors of cyan, magenta, and yellow.

In the driving method of the liquid crystal display device according toan embodiment of the present invention, the light which is emitted fromthe backlight portion may be light emitted from a light-emitting diode.

In the driving method of the liquid crystal display device according toan embodiment of the present invention, a plurality of pixels of theliquid crystal display device may include a liquid crystal element and atransistor for controlling the liquid crystal element, and the liquidcrystal element may include a liquid crystal material exhibiting a bluephase.

In the driving method of the liquid crystal display device according toan embodiment of the present invention, a semiconductor layer of thetransistor may include an oxide semiconductor.

An embodiment of the present invention can realize lower powerconsumption in displaying a 3D image with field sequential driving.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are perspective views illustrating one embodiment of thepresent invention.

FIG. 2 is a timing chart according to one embodiment of the presentinvention.

FIG. 3 is a timing chart according to one embodiment of the presentinvention.

FIG. 4 is a timing chart according to one embodiment of the presentinvention.

FIG. 5 is a timing chart according to one embodiment of the presentinvention.

FIG. 6 is a timing chart according to one embodiment of the presentinvention.

FIG. 7 is a timing chart according to one embodiment of the presentinvention.

FIG. 8 is a timing chart according to one embodiment of the presentinvention.

FIGS. 9A to 9C are drawings for illustrating one embodiment of thepresent invention.

FIGS. 10A to 10D are each a cross-sectional view of one embodiment ofthe present invention.

FIG. 11 is a top view of one embodiment of the present invention.

FIG. 12 is a cross-sectional view of one embodiment of the presentinvention.

FIG. 13 is a top view of one embodiment of the present invention.

FIG. 14 is a cross-sectional view of one embodiment of the presentinvention.

FIGS. 15A to 15E are cross-sectional views of one embodiment of thepresent invention.

FIGS. 16A to 16D are diagrams each illustrating an electronic device ofone embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. However, the present inventioncan be carried out in many different modes, and it is easily understoodby those skilled in the art that modes and details of the presentinvention can be modified in various ways without departing from thepurpose and the scope of the present invention. Therefore, thisinvention is not interpreted as being limited to the description of theembodiments below. Note that identical portions or portions having thesame function in all drawings illustrating the structure of theinvention that are described below are denoted by the same referencenumerals.

Note that the size, the thickness of a layer, distortion of the waveformof a signal, and a region of each structure illustrated in the drawingsand the like in the embodiments are exaggerated for simplicity in somecases. Therefore, embodiments of the present invention are not limitedto such scales.

Note that terms “first”, “second”, “third” to “Nth” (N is a naturalnumber) employed in this specification are used in order to avoidconfusion between components and do not set a limitation on number.

Embodiment 1

In order to describe a driving method of a liquid crystal displaydevice, a structure of a liquid crystal display device capable ofdisplaying a 3D (three-dimensional or stereoscopic) image is explainedfirst.

In the driving method of the liquid crystal display device in thisembodiment, a stereoscopic image can be perceived with a liquid crystaldisplay device switching an image for a left eye and an image for aright eye to display the image for the left eye or the image for theright eye, and a pair of glasses having a switching means with which theimages are switched in synchronization with display of the image for theleft eye or the image for the right eye in order that the left or righteye of a viewer may selectively perceive the image for the left eye orthe image for the right eye (hereinafter, abbreviated to glasses).

For specific description, FIG. 1A shows a perspective view in which aliquid crystal display device 101 is connected to glasses 102 with acable 103. In the glasses 102, shutters of a left panel 104A for a lefteye and a right panel 104B for a right eye are alternately opened andclosed. Synchronized with opening and closing of the shutters, the imagefor the left eye and the image for the right eye are alternatelydisplayed on the liquid crystal display device 101. Thus, a viewer canperceive the image on the liquid crystal display device 101 as a 3Dimage.

The shutters provided for the left panel 104A for the left eye and theright panel 104B for the right eye of the glasses 102 may include aliquid crystal element having electrodes with a liquid crystal materialprovided therebetween. The liquid crystal material may includeferroelectric liquid crystal, a liquid crystal material which exhibits ablue phase, or the like. Note that by opening the shutter, light from abacklight portion in accordance with an image of the liquid crystaldisplay device 101 is led to the left or right eye of the viewer. Inaddition, by closing the shutter, light from the backlight portion tothe left or right eye of the viewer is blocked.

Although in FIG. 1A, the cable 103 is shown as a means for electricalconnection between the liquid crystal display device 101 and the glasses102, signals between the liquid crystal display device 101 and theglasses 102 may be synchronized by a wireless signal. In a structure inwhich the signals between the liquid crystal display device 101 and theglasses 102 are synchronized by the wireless signal, the glasses 102 maybe provided with a secondary battery, such as a lithium-ion battery, asan electrical storage means.

FIG. 1B shows a block diagram of a main structure of the liquid crystaldisplay device 101 and the glasses 102.

The liquid crystal display device 101 illustrated in FIG. 1B includes adisplay control circuit 115, a display portion 111, a timing generator116, a source line driver circuit 113, a backlight portion 114, and agate line driver circuit 112.

The display control circuit 115 switches and outputs a signal fordisplaying a 2D (two-dimensional, or plane) image and a signal fordisplaying a 3D image, in accordance with operation with an externaloperation means 117 such as a keyboard or a remote controller.Specifically, in the case of displaying a 3D image, the display controlcircuit 115 supplies the following to the backlight portion 114, thesource line driver circuit 113, and the gate line driver circuit 112:signals for displaying an image for the left eye and an image for theright eye with field sequential driving (for example, a start pulse, aclock signal, image signals of the image for the left eye and the imagefor the right eye, and a control signal of a backlight).

Note that in the case where the image for the left eye and the image forthe right eye are displayed with the field sequential driving, light ofa plurality of colors, which is emitted from the backlight portion, isswitched within a predetermined period and is delivered to the left andright eyes of the viewer, and the viewer perceive color display in amixed color. Note that when the light emitted from the backlight portionis a light emitted from a light-emitting diode, the size of a lightsource can be reduced, power consumption can be reduced, and life can belong. At that time, the backlight portion is provided withlight-emitting diodes which are the light sources corresponding tocolors of R, G, B, and the like.

Note that the display control circuit 115 is supplied with the followingfrom the timing generator 116: a signal for synchronizing the imagesignals of the image for the left eye and the image for the right eyewith the left panel 104A for the left eye and the right panel 104B forthe right eye of the glasses 102. Note that the timing generator 116supplies the following to the glasses 102: a signal for synchronizingdisplay of the image for the left eye and movement of the shutter of theleft panel 104A for the left eye, and a signal for synchronizing displayof the image for the right eye and movement of the shutter of the rightpanel 104B for the right eye. In other words, a synchronization signal119A of the image for the left eye is supplied to the display controlcircuit 115, and a synchronization signal 118A for opening the shutterof the left panel 104A for the left eye is input to the left panel 104Afor the left eye. Further, a synchronization signal 119B of the imagefor the right eye is supplied to the display control circuit 115, and asynchronization signal 118B for opening the shutter of the right panel104B for the right eye is input to the right panel 104B for the righteye.

Next, FIG. 2 shows a timing chart of the structure in FIGS. 1A and 1B,and the driving method of the liquid crystal display device of thisembodiment is described.

FIG. 2 shows a timing chart in a stereoscopic image display period 200which is a period in which one image for the left eye and one image forthe right eye are displayed so that a viewer can perceive a stereoscopicimage. With a series of different stereoscopic image display periods200, the viewer can perceive a stereoscopic moving image. Thestereoscopic image display period 200 has an R-lighting period 201 forperforming display with a red (R) backlight, a G-lighting period 202 forperforming display with a green (G) backlight, and a B-lighting period203 for performing display with a blue (B) backlight. The R-lightingperiod 201 has an R-lighting period 211 for a left eye and an R-lightingperiod 212 for a right eye. The G-lighting period 202 has a G-lightingperiod 221 for the left eye and a G-lighting period 222 for the righteye. The B-lighting period 203 has a B-lighting period 231 for the lefteye and a B-lighting period 232 for the right eye.

In the R-lighting period 211 for the left eye, a writing operation 241to sequentially write image signals to gate signal lines (scan lines)from the first to the last row is performed in an R-writing period LRfor the left eye, and a period for controlling the backlight portion toturn on the R backlight is provided after the R-writing period LR forthe left eye. Further, the R-lighting period 211 for the left eye is aperiod in which the shutter of the left lens which is provided for theglasses so that the left eye of a viewer can perceive the image for theleft eye is opened, and a period in which the shutter of the right lenswhich is provided for the glasses so that the right eye of the viewercan perceive the image for the right eye is closed.

Then, in the R-lighting period 212 for the right eye, a writingoperation 242 to sequentially write image signals to gate signal lines(scan lines) from the first to the last row is performed in an R-writingperiod RR for the right eye, and a period for controlling the backlightportion to turn on the R backlight is provided after the R-writingperiod RR for a right eye. Further, the R-lighting period 212 for theright eye is a period in which the shutter of the right lens which isprovided for the glasses so that the right eye of a viewer can perceivethe image for the right eye is opened, and a period in which the shutterof the left lens which is provided for the glasses so that the left eyeof the viewer can perceive the image for the left eye is closed.

Next, in the G-lighting period 221 for the left eye, an image signalwriting operation and control of the G backlight to be turned on areperformed as in the R-lighting period 211 for the left eye. Further, theG-lighting period 221 for the left eye is a period in which the shutterof the left lens is opened and the shutter of the right lens is closed.Then, in the G-lighting period 222 for the right eye, an image signalwriting operation and control of the G backlight to be turned on areperformed as in the R-lighting period 212 for the right eye. Further,the G-lighting period 222 for the right eye is a period in which theshutter of the right lens is opened and the shutter of the left lens isclosed.

Next, in the B-lighting period 231 for the left eye, an image signalwriting operation and control of the B backlight to be turned on areperformed as in the R-lighting period 211 for the left eye. Further, theB-lighting period 231 for the left eye is a period in which the shutterof the left lens is opened and the shutter of the right lens is closed.Then, in the B-lighting period 232 for the right eye, an image signalwriting operation and control of the B backlight to be turned on areperformed as in the R-lighting period 212 for the right eye. Further,the B-lighting period 232 for the right eye is a period in which theshutter of the right lens is opened and the shutter of the left lens isclosed.

Note that in description in this specification, “the shutter is opened”or “the shutter is closed” corresponds to controlling voltage applied toa liquid crystal element of the left lens or the right lens. Thus, indrawings illustrating timing charts, opening the shutter is denoted by“ON” and closing the shutter is denoted by “OFF”.

In the field sequential driving, the viewer perceives the image for theleft eye or the image for the right eye while sequential lighting of theR, G and B backlights are repeated, as described in Patent Document 1.Therefore, in the structure in which a stereoscopic image is perceivedby alternate and sequential display of the images for the left eye andthe images for the right eye, it is necessary to turn on the Rbacklight, the G backlight, and the B backlight one after the other athigh rate for each of the images for the left and right eyes;accordingly, there is a concern about an increase in power consumption.

On the other hand, in the timing chart of this embodiment in FIG. 2, thelighting period for the left eye and the lighting period for the righteye are sequentially provided in each of the R-lighting period 201, theG-lighting period 202, and the B-lighting period 203. The R lighting,the G lighting, and the B lighting are sequentially performed in each ofdisplays of the images for the left eye and the right eye; thus, thenumber of switching operations of backlight lighting, such as switchingfrom the R lighting to the G lighting, and switching from the G lightingto the B lighting, can be reduced. Therefore, power consumption neededfor switching lightings of the backlights can be reduced. The larger theliquid crystal display device and the number of light sources of thebacklight portion become, the more suitable the structure of thisembodiment becomes.

Note that the timing chart in FIG. 2 shows a structure in which in theR-lighting period 201, the G-lighting period 202, and the B-lightingperiod 203, the backlight is turned off in an image signal writingoperation in a period in which the lighting period for the left eye andthe lighting period for the right eye are sequential, that is, forexample, in the R-writing period RR for the right eye. That is astructure in which perception of a writing operation to each pixel isprevented, which may be caused when the backlight is on. Note that inthe structure of this embodiment, in the case where opening and closingthe shutters of the left panel for the left eye and the right panel forthe right eye of the glasses are switched, the structure is realized inwhich the backlight is kept on, without the backlight turned off, in theimage signal writing operation in the period in which the lightingperiod for the left eye and the lighting period for the right eye aresequential, that is, for example, in the R-writing period RR for theright eye. With the structure in which the backlight is kept on in theB-lighting period 201, the G-lighting period 202, and the B-lightingperiod 203, power consumption can be reduced in comparison with thestructure in which lighting of the R, G, and B backlights is repeatedand the structure of the timing chart in FIG. 2.

Note that in the case of displaying an image with the field sequentialdriving, it is preferable that lighting periods of R, G, and B in thestereoscopic image display period 200 be short so that display defectsdue to color breaking is hardly visible. Therefore, in view ofenlargement of the liquid crystal display device, a transistor with asemiconductor layer including an oxide semiconductor is preferable as atransistor provided for each pixel of the liquid crystal display devicefor shortening the lighting period. A transistor with a semiconductorlayer including an oxide semiconductor can be driven at high speedbecause of its high field effect mobility in comparison with atransistor with a semiconductor layer including amorphous silicon, canbe fabricated without a process such as laser irradiation, and canrealize formation of a transistor over a large substrate; accordingly,the transistor with a semiconductor layer including an oxidesemiconductor is preferable. Further, a liquid crystal material of aliquid crystal element provided in the liquid crystal display device ispreferably a liquid crystal material which can respond to high-speeddriving. For example, as a liquid crystal material, ferroelectric liquidcrystal, a liquid crystal material which exhibits a blue phase, and aliquid crystal material which exhibits a nematic phase with a narrowcell gap are preferably used.

Note that the structure in which three colors of R, G, and B are used ascolors of light sources of the backlights is described in FIG. 2;however, colors such as Y (yellow), C (cyan), and M (magenta) may beused. A structure in which as another color in addition to R, G, and B,white (hereinafter, sometimes abbreviated to W) obtained by lighting R,G, and B at the same time is used is shown in FIG. 5 and described inaddition to the structure in FIG. 2.

A timing chart in FIG. 5 is different from that in FIG. 2 in that aW-lighting period 204 is provided in the stereoscopic image displayperiod 200. The W-lighting period 204 has a W-lighting period 251 for aleft eye and a W-lighting period 252 for a right eye. In the timingchart in FIG. 5, in the W-lighting period 251 for the left eye whichcomes after the B-lighting period 203, an image signal writing operationis performed as in the R-lighting period 211 for the left eye, andcontrol of the R, G, and B backlights to be turned on are performed.Further, the G-lighting period 221 for the left eye is a period in whichthe shutter of the left lens is opened and the shutter of the right lensis closed. Then, in the W-lighting period 252 for the right eye, animage signal writing operation and control of the R, G, and B backlightsto be turned on are performed as in the R-lighting period 212 for theright eye. Further, the W-lighting period 252 for the right eye is aperiod in which the shutter of the right lens is opened and the shutterof the left lens is closed.

A color corresponding to white in addition to R, G, and B is added as inthe timing chart in FIG. 5, so that display defects due to colorbreaking which occurs in displaying an image with the field sequentialdriving can hardly be visible. Therefore, reduction in color breaking inthe field sequential driving can be achieved.

In addition to the structure described with reference to FIG. 5 in whichwhite obtained by lighting R, G, and B at the same time is used, thestructure in which a light source capable of emitting white light aloneis used is described with reference to FIG. 6.

A timing chart in FIG. 6 is different from that in FIG. 5 in thatcontrol of turning on a W-light source separately provided in additionto light sources of R, G, and B is performed in the W-lighting period204 which comes after the B-lighting period 203. As in the structure inFIG. 6, the W-light source separately provided in addition to lightsources of R, G, and B is turned on in the W-lighting period 251 for theleft eye and the W-lighting period 252 for the right eye, so that thenumber of light sources which are turned on can be reduced in comparisonwith the structure in which R, G, and B are lit at the same time, andpower consumption can also be reduced.

FIG. 3 shows a timing chart different from that in FIG. 2.

FIG. 3 shows the timing chart in the stereoscopic image display period200, as in FIG. 2. The stereoscopic image display period 200 has theR-lighting period 201, the G-lighting period 202, and the B-lightingperiod 203, as in FIG. 2. The R-lighting period 201 has the R-lightingperiod 211 for a left eye and the R-lighting period 212 for a right eye,as in FIG. 2. The G-lighting period 202 has the G-lighting period 221for a left eye and the G-lighting period 222 for a right eye, as in FIG.2. The B-lighting period 203 has the B-lighting period 231 for a lefteye and the B-lighting period 232 for a right eye, as in FIG. 2.

In the R-writing period LR for the left eye of the R-lighting period 211for the left eye, the writing operation 241 to sequentially write imagesignals to gate signal lines (scan lines) from the first to the last rowis performed, and the backlight portion is controlled so as to turnedoff the R backlight. Further, the R-writing period LR for the left eyeis a period in which the shutters of the left panel for the left eye andthe right panel for the right eye are closed. After the image signalwriting operation 241 is performed up to the last row, the R backlightis turned on, the shutter of the left lens is opened, and the shutter ofthe right lens is closed.

Note that in the structure in FIG. 3, the shutters of the left panel forthe left eye and the right panel for the right eye are closed in theimage signal writing operation of the R-writing period LR for the lefteye. Accordingly, the backlight portion may be controlled so that the Rbacklight is turned on in the image signal writing operation of theR-writing period LR for the left eye.

Then, in the R-writing period RR for the right eye of the R-lightingperiod 212 for the right eye, the writing operation 242 to sequentiallywrite image signals to the gate signal lines (scan lines) from the firstto the last row is performed, and the backlight portion is controlled soas to turn on the R backlight. Further, the R-writing period RR for theright eye is a period in which the shutters of the left panel for theleft eye and the right panel for the right eye are closed. After theimage signal writing operation 242 is performed up to the last row, theR backlight is turned on, the shutter of the right lens is opened, andthe shutter of the left lens is closed.

Note that in the structure in FIG. 3, the shutters of the left panel forthe left eye and the right panel for the right eye are closed in theimage signal writing operation of the R-writing period RR for the righteye. Accordingly, the backlight portion can be controlled so that the Rbacklight is turned on in the image signal writing operation of theR-writing period RR for the right eye. In other words, with thestructure of the timing chart in FIG. 3, the number of operations forswitching on and off the R backlight in the R-lighting period 201 can bereduced. Therefore, power consumption needed for switching lightings ofthe backlights can be reduced. The larger the liquid crystal displaydevice and the number of light sources of the backlight portion become,the more suitable the structure of this embodiment becomes.

Next, in the G-lighting period 221 for the left eye, an image signalwriting operation and control of the G backlight to be turned on areperformed as in the R-lighting period 211 for the left eye. Further, theshutters of the left panel for the left eye and the right panel for theright eye are closed in the image signal writing operation. After thewriting operation is finished, the G-lighting period 221 for the lefteye is a period in which the shutter of the left lens is opened and theshutter of the right lens is closed. Then, in the G-lighting period 222for the right eye, an image signal writing operation and control of theG backlight to be turned on are performed as in the R-lighting period212 for the right eye. Further, the shutters of the left panel for theleft eye and the right panel for the right eye are closed in the imagesignal writing operation. After the writing operation is finished, theG-lighting period 222 for the right eye is a period in which the shutterof the right lens is opened and the shutter of the left lens is closed.

Next, in the B-lighting period 231 for the left eye, an image signalwriting operation and control of the G backlight to be turned on areperformed as in the R-lighting period 211 for the left eye. Further, theshutters of the left panel for the left eye and the right panel for theright eye are closed in the image signal writing operation. After thewriting operation is finished, the B-lighting period 231 for the lefteye is a period in which the shutter of the left lens is opened and theshutter of the right lens is closed. Then, in the B-lighting period 232for the right eye, an image signal writing operation and control of theB backlight to be turned on are performed as in the R-lighting period212 for the right eye. Further, the shutters of the left panel for theleft eye and the right panel for the right eye are closed in the imagesignal writing operation. After the writing operation is finished, theB-lighting period 232 for the right eye is a period in which the shutterof the right lens is opened and the shutter of the left lens is closed.

Note that for the structure in FIG. 3, the structure can be used inwhich as another color in addition to R, G, and B, white obtained bylighting R, G, and B at the same time is used, as in FIG. 5. FIG. 7shows a specific timing chart. A color corresponding to white inaddition to R, G, and B is added as in the timing chart in FIG. 7, sothat display defects due to color breaking which occurs in displaying animage with the field sequential driving can hardly be visible, inaddition to the effect of the structure in FIG. 3. Therefore, reductionin color breaking in the field sequential driving can be achieved.

In addition to the structure described in FIG. 7 in which white obtainedby lighting R, G, and B at the same time is used, the structure in whicha light source capable of emitting white light alone can be obtained isused is described with reference to FIG. 8. As in the structure in FIG.8, the W-light source separately provided in addition to light sourcesof R, G, and B is turned on in the W-lighting period 251 for the lefteye and the W-lighting period 252 for the right eye, so that the numberof light sources which are turned on can be reduced in comparison withthe structure in which R, G, and B are lit at the same time, and powerconsumption can also be reduced.

Then, FIG. 4 shows a diagram illustrating how the left and right eyes ofthe viewer perceive the images displayed in the stereoscopic imagedisplay period 200 with the structure illustrated in the timing chartsof FIG. 2 and FIG. 3.

As shown in FIG. 4, in the timing charts of FIG. 2 and FIG. 3, in thestereoscopic image display period 200, the left eye of the viewerperceives an image displayed by lighting the R backlight, an imagedisplayed by lighting the G backlight, and an image displayed bylighting the B backlight as an image in a mixed color (in FIG. 4, theimage for the left eye), and the right eye of the viewer perceives animage displayed by the R backlight, an image displayed by the Gbacklight, and an image displayed by the B backlight as an image in amixed color (in FIG. 4, the image for the right eye).

In FIG. 4, the left eye of the viewer perceives the image for the lefteye through the following periods: the R-writing period LR for the lefteye in which the image not perceived (BLANK in FIG. 4), an R-lightingperiod (R_L in FIG. 4), a period in which the image is not perceivedbecause of display of the image for the right eye (BLANK in FIG. 4), aG-writing period LG for the left eye in which the image is not perceived(BLANK in FIG. 4), the G-lighting period (R_L in FIG. 4), a period inwhich the image is not perceived because of display of the image for theright eye (BLANK in FIG. 4), a B-writing period LB for the left eye inwhich the image is not perceived (BLANK in FIG. 4), the B-lightingperiod (B_L in FIG. 4), and a period in which the image is not perceivedbecause of display of the image for the right eye (BLANK in FIG. 4). Inother words, the left eye of the viewer perceives the image for the lefteye in a mixed color by switching light which is emitted from thebacklight portion, and which corresponds to a plurality of colors (RGB),within a predetermined period.

In FIG. 4, the right eye of the viewer perceives the image for the righteye through the following periods: a period in which the image is notperceived because of display of the image for the right eye (BLANK inFIG. 4), the R-writing period RR for the right eye in which the imagenot perceived (BLANK in FIG. 4), the R-lighting period (R_L in FIG. 4),a period in which the image is not perceived because of display of theimage for the left eye (BLANK in FIG. 4), a G-writing period RG for theright eye in which the image is not perceived (BLANK in FIG. 4), theG-lighting period (R_L in FIG. 4), a period in which the image is notperceived because of display of the image for the left eye (BLANK inFIG. 4), a B-writing period RB for the right eye in which the image isnot perceived (BLANK in FIG. 4), and the B-lighting period (B_L in FIG.4). In other words, the left eye of the viewer perceives the image forthe right eye in a mixed color by switching light which is emitted fromthe backlight portion, and which corresponds to a plurality of colors(RGB), within a predetermined period.

Within the stereoscopic image display period 200, as illustrated in FIG.4, the left and right eyes of the viewer perceive an image displayed bylighting the R backlight, an image displayed by lighting the Gbacklight, and an image displayed by lighting the B backlight. As shownin FIG. 4, in the timing charts of FIG. 2 and FIG. 3, a period in whichthe backlight emits light which corresponds to a plurality of colors(RGB) is provided so that colors are mixed to be perceived as display ofthe image, and a period in which display of the image is not perceived(BLANK in FIG. 4) is provided in the period in which the backlights emitlight which corresponds to a plurality of colors (RGB). Thus, with thedriving method of this embodiment, the viewer perceives display of theimage in which a black image is appropriately inserted; therefore, aresidual image, blur, color-breaking, and the like in displaying amoving image can be suppressed, which results in improvement of movingimage display characteristics.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 2

In this embodiment, an example of a structure of a backlight portion(also referred to as a backlight or a backlight unit) which can be usedfor the liquid crystal display device disclosed in this specificationwill be described with reference to FIGS. 9A to 9C.

FIG. 9A illustrates an example of a liquid crystal display deviceincluding a what is called edge-light type backlight portion 5201 and adisplay panel 5207. An edge-light type has a structure in which a lightsource is provided at an end of the backlight portion and light from thelight source is emitted from the entire light-emitting surface.

The backlight portion 5201 includes a diffusion plate 5202 (alsoreferred to as a diffusion sheet), a light guide plate 5203, areflection plate 5204, a lamp reflector 5205, and a light source 5206.Note that the backlight portion 5201 may also include a luminanceimprovement film or the like.

The light source 5206 has a function of emitting light with differentcolors (RGB) as necessary. For example, as the light source 5206, a coldcathode fluorescent lamp (CCFL) provided with a color filter, a lightemitting diode, an EL element, or the like is used.

FIG. 9B illustrates a detailed structure of an edge-light type backlightportion. Note that description of the diffusion plate, the light guideplate, the reflection plate, and the like is omitted.

The backlight portion 5201 illustrated in FIG. 9B has a structure inwhich light-emitting diodes (LEDs) 5223R, 5223G, and 5223B correspondingto R, G, and B, respectively are used as light sources. Thelight-emitting diodes (LEDs) 5223R, 5223G, and 5223B corresponding to R,G, and B, respectively are provided at a predetermined interval. Inaddition, a lamp reflector 5222 is provided to efficiently reflect lightfrom the light-emitting diodes (LEDs) 5223R, 5223G, and 5223Bcorresponding to R, G, and B, respectively. Note that a diode whichemits white light may be provided as a light source in addition to thelight-emitting diodes (LEDs) 5223R, 5223G, and 5223B corresponding to R,G, and B, respectively.

FIG. 9C illustrates an example of a liquid crystal display deviceincluding a what is called direct-below-type backlight portion and aliquid crystal panel. A direct-below type has a structure in which alight source is provided directly under a light-emitting surface andlight from the light source is emitted from the entire light-emittingsurface.

A backlight portion 5290 includes a diffusion plate 5291, alight-shielding portion 5292, a lamp reflector 5293, a liquid crystalpanel 5295, and light-emitting diodes (LEDs) 5294R, 5294G and 5294Bcorresponding to R, G, and B, respectively.

Note that in the what is called direct-below-type backlight portion, anEL element which is a light-emitting element is used instead of alight-emitting diode (LED) serving as a light source, so that thethickness of the backlight portion can be reduced.

Note that the backlight portion described in FIGS. 9A to 9C may have astructure in which luminance is adjusted. For example, luminance isadjusted in accordance with illuminance around the liquid crystaldisplay device or luminance is adjusted in accordance with an imagesignal for display may be employed.

This embodiment can be combined with any of the other embodiments asappropriate.

Embodiment 3

In this embodiment, an example of a transistor that can be applied to aliquid crystal display device disclosed in this specification will bedescribed. There is no particular limitation on a structure of thetransistor that can be applied to the liquid crystal display devicedisclosed in this specification. For example, a staggered transistor, aplanar transistor, or the like having a top-gate structure in which agate electrode is provided above an oxide semiconductor layer with agate insulating layer interposed or a bottom-gate structure in which agate electrode is provided below an oxide semiconductor layer with agate insulating layer interposed, can be used. The transistor may have asingle gate structure including one channel formation region, a doublegate structure including two channel formation regions, or a triple gatestructure including three channel formation regions. Alternatively, thetransistor may have a dual gate structure including two gate electrodelayers provided over and below a channel region with a gate insulatinglayer interposed. FIGS. 10A to 10D illustrate examples ofcross-sectional structures of transistors. Each of the transistorsillustrated in FIGS. 10A to 10D includes an oxide semiconductor as asemiconductor layer. An advantage of using an oxide semiconductor isthat high field-effect mobility (the maximum value is 5 cm²/Vsec orhigher, preferably in the range of 10 cm²/Vsec to 150 cm²/Vsec) can beobtained when a transistor is on, and low off-state current (forexample, off-state current per channel width is lower than 1 aA/μm,preferably lower than 10 zA/μm and lower than 100 zA/μm at 85° C.) canbe obtained when the transistor is off.

A transistor 410 illustrated in FIG. 10A is one of bottom-gatetransistors and is also referred to as an inverted staggered transistor.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating layer 402, anoxide semiconductor layer 403, a source electrode layer 405 a, and adrain electrode layer 405 b. An insulating film 407 is provided to coverthe transistor 410 and be stacked over the oxide semiconductor layer403. Further, a protective insulating layer 409 is formed over theinsulating film 407.

A transistor 420 illustrated in FIG. 10B is one of bottom-gatetransistors referred to as a channel-protective type (also referred toas a channel-stop type) and is also referred to as an inverted staggeredtransistor.

The transistor 420 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating layer 402,the oxide semiconductor layer 403, an insulating layer 427 functioningas a channel protective layer covering a channel formation region of theoxide semiconductor layer 403, the source electrode layer 405 a, and thedrain electrode layer 405 b. Further, the protective insulating layer409 is formed to cover the transistor 420.

A transistor 430 illustrated in FIG. 10C is a bottom-gate transistor andincludes, over the substrate 400 having an insulating surface, the gateelectrode layer 401, the gate insulating layer 402, the source electrodelayer 405 a, the drain electrode layer 405 b, and the oxidesemiconductor layer 403. The insulating film 407 is provided to coverthe transistor 430 and to be in contact with the oxide semiconductorlayer 403. Further, the protective insulating layer 409 is formed overthe insulating film 407.

In the transistor 430, the gate insulating layer 402 is provided overand in contact with the substrate 400 and the gate electrode layer 401;the source electrode layer 405 a and the drain electrode layer 405 b areprovided over and in contact with the gate insulating layer 402. Theoxide semiconductor layer 403 is provided over the gate insulating layer402, the source electrode layer 405 a, and the drain electrode layer 405b.

A transistor 440 illustrated in FIG. 10D is one of top-gate transistors.The transistor 440 includes, over the substrate 400 having an insulatingsurface, an insulating layer 437, the oxide semiconductor layer 403, thesource electrode layer 405 a, the drain electrode layer 405 b, the gateinsulating layer 402, and the gate electrode layer 401. A wiring layer436 a and a wiring layer 436 b are provided in contact with andelectrically connected to the source electrode layer 405 a and the drainelectrode layer 405 b respectively.

In this embodiment, the oxide semiconductor layer 403 is used as asemiconductor layer as described above. As an oxide semiconductor usedfor the oxide semiconductor layer 403, the following metal oxides can beused: a four-component metal oxide such as an In—Sn—Ga—Zn—O-based oxidesemiconductor; a three-component metal oxide such as an In—Ga—Zn—O-basedoxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga-Zn—O-based oxide semiconductor, and aSn—Al—Zn—O-based oxide semiconductor; a two-component metal oxide suchas an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, and anIn—Mg—O-based oxide semiconductor; an In—O-based oxide semiconductor; aSn—O-based oxide semiconductor; a Zn—O-based oxide semiconductor; anIn—Ga—O-based oxide semiconductor. In addition, SiO₂ may be contained inthe above oxide semiconductor. Here, for example, an In—Ga—Zn—O-basedoxide semiconductor means an oxide film containing indium (In), gallium(Ga), and zinc (Zn), and there is no particular limitation on thecomposition ratio thereof The In—Ga—Zn—O-based oxide semiconductor maycontain an element other than In, Ga, and Zn.

As the oxide semiconductor layer 403, a thin film represented by achemical formula of InM₃(ZnO)_(m) (m>0) can be used. Here, M representsone or more metal elements selected from Zn, Ga, Al, Mn, and Co. Forexample, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In each of the transistors 410, 420, 430, and 440 including the oxidesemiconductor layer 403, the current value in an off state (off-statecurrent value) can be reduced. Thus, in a pixel, a capacitor for holdingan electric signal such as an image signal can be designed to besmaller. Accordingly, the aperture ratio of the pixel can be increased,so that power consumption can be suppressed.

In addition, each of the transistors 410, 420, 430, and 440 includingthe oxide semiconductor layer 403 can operate at high speed becauserelatively high field-effect mobility can be obtained. It has been foundthat color breaking is reduced by increase in frame frequency.Consequently, when the above transistors are used in a pixel portion ofa liquid crystal display device, color breaking can be suppressed byincrease in frame frequency and high-quality images can be obtained. Inaddition, since the above transistors can be provided in each of adriver circuit portion and a pixel portion over one substrate, thenumber of components of the liquid crystal display device can bereduced.

There is no limitation on a substrate that can be applied to thesubstrate 400 having an insulating surface; however, a glass substratesuch as a glass substrate made of barium borosilicate glass oraluminosilicate glass is used.

In the bottom-gate transistors 410, 420, and 430, an insulating filmserving as a base film may be provided between the substrate and thegate electrode layer. The base film has a function of preventingdiffusion of an impurity element from the substrate, and can be formedto have a stacked-layer structure using one or more of a silicon nitridefilm, a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

The gate electrode layer 401 can be formed to have a single-layerstructure or a stacked-layer structure using any of a metal materialsuch as molybdenum, titanium, chromium, tantalum, tungsten, aluminum,copper, neodymium, or scandium, or an alloy material which contains anyof these materials as its main component.

The gate insulating layer 402 can be formed with a single-layerstructure or a stacked structure using any of a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, an aluminum oxide layer, an aluminum nitride layer, analuminum oxynitride layer, an aluminum nitride oxide layer, and ahafnium oxide layer by a plasma CVD method, a sputtering method, or thelike. For example, a silicon nitride layer (SiN_(y) (y>0)) having athickness of 50 nm to 200 nm inclusive is formed as a first gateinsulating layer by a plasma CVD method, and a silicon oxide layer(SiO_(x) (x>0)) having a thickness of 5 nm to 300 nm inclusive is formedas a second gate insulating layer over the first gate insulating layer,so that a gate insulating layer with a total thickness of 200 nm isformed.

As a conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b, for example, a metal film containing anelement selected from Al, Cr, Cu, Ta, Ti, Mo, and W and a metal nitridefilm containing the above elements as its main component (a titaniumnitride film, a molybdenum nitride film, and a tungsten nitride film)can be used. A metal film having a high melting point of Ti, Mo, W, orthe like or a metal nitride film of these elements (a titanium nitridefilm, a molybdenum nitride film, and a tungsten nitride film) may bestacked on one of or both of a lower side or an upper side of a metalfilm of Al, Cu, or the like.

A conductive film functioning as the wiring layer 436 a and the wiringlayer 436 b connected to the source electrode layer 405 a and the drainelectrode layer 405 b can be formed using a material similar to that ofthe source electrode layer 405 a and the drain electrode layer 405 b.

The conductive film to be the source electrode layer 405 a and the drainelectrode layer 405 b (including a wiring layer formed using the samelayer as the source electrode layer 405 a and the drain electrode layer405 b) may be formed using conductive metal oxide. As the conductivemetal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO),an alloy of indium oxide and tin oxide (In₂O₃—SnO₂, referred to as ITO),an alloy of indium oxide and zinc oxide (In₂O₃—ZnO), and such a metaloxide material containing silicon oxide can be used.

As the insulating films 407 and 427 provided over the oxidesemiconductor layer, and the insulating film 437 provided under theoxide semiconductor layer, an inorganic insulating film such as asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, or the like can be typically used.

For the protective insulating layer 409 provided over the oxidesemiconductor layer, an inorganic insulating film such as a siliconnitride film, an aluminum nitride film, a silicon nitride oxide film, oran aluminum nitride oxide film can be used.

Further, a planarization insulating film may be formed over theprotective insulating layer 409 so that surface roughness due to thetransistor is reduced. As the planarization insulating film, an organicmaterial such as polyimide, acrylic, and benzocyclobutene can be used.Besides the above organic materials, a low-dielectric constant material(a low-k material) or the like can be used. Note that the planarizationinsulating film may be formed by stacking a plurality of insulatingfilms formed of these materials.

An example of a pixel in a liquid crystal display device using such atransistor is illustrated in FIG. 11 and FIG. 12. FIG. 11 is a plan viewof the pixel and FIG. 12 is a cross-sectional view taken along a lineA-B shown in FIG. 11. Note that FIG. 11 illustrates a plan view of thesubstrate 400 over which the transistor 410 is formed, and FIG. 12illustrates a structure in which a counter substrate 416 and a liquidcrystal layer 414 are formed in addition to a structure of the substrate400 over which a transistor 410 is formed. The following descriptionwill be given with reference to both FIG. 11 and FIG. 12.

The transistor 410 has the same structure as that of FIG. 10A, andincludes the gate electrode layer 401, the gate insulating layer 402,and the oxide semiconductor layer 403. In a pixel, the gate electrodelayer 401 is formed to extend in one direction. The oxide semiconductorlayer 403 is provided to overlap with the gate electrode layer 401 withthe gate insulating layer 402 interposed therebetween. The sourceelectrode layer 405 a and the drain electrode layer 405 b are providedover the oxide semiconductor layer 403 (note that here, the terms “thesource electrode layer 405 a” and “the drain electrode layer 405 b” areused just for convenience in order to distinguish electrodes in thetransistor 410 between source and drain). The source electrode layer 405a is extended in a direction to intersect with the gate electrode layer401. A first pixel electrode 411 is provided over the protectiveinsulating layer 409, and the first pixel electrode 411 is connected tothe drain electrode layer 405 b through a contact hole 412. The firstpixel electrode 411 is formed from a transparent electrode material suchas indium tin oxide, zinc oxide, or tin oxide.

Further, a common wiring layer 417 and a common electrode layer 418which are provided in the same layer as the gate electrode layer 401 areprovided, and a second pixel electrode 419 is provided. Note thatalthough not particularly illustrated in FIG. 11 and FIG. 12, a storagecapacitor may be provided in the pixel as necessary.

The first pixel electrode 411 and the second pixel electrode 419 areformed to have a comb shape and can control alignment of liquid crystalin accordance with the electric field generated between the electrodes.Such a structure is applied to an IPS (in-plane switching) mode. The IPSmode is a mode of controlling alignment of liquid crystal molecules of aliquid crystal panel. The IPS mode is a mode in which an electrode isprovided so as to be horizontal to the substrate and liquid crystalmolecules are horizontal.

A counter electrode 415 is provided on the counter substrate 416 side.The liquid crystal layer 414 is provided between the substrate 400 andthe counter substrate 416. An alignment film 413 is provided to be incontact with the liquid crystal layer 414. Alignment treatment for thealignment film 413 is made by an optical alignment method or a rubbingmethod. As a liquid crystal phase of the liquid crystal layer 414, ablue phase, or the like can be used.

The following components constitutes one unit: the transistor 410 inwhich the oxide semiconductor layer 403 is provided to overlap with thegate electrode layer 401 with the gate insulating layer 402 interposedtherebetween; the first pixel electrode 411 which is connected to thesource side or the drain side of the transistor 410 and drives liquidcrystal; the second pixel electrode 419 provided to face the first pixelelectrode 411; and the liquid crystal layer 414. A pixel can beconstituted by the one or more of units, and the pixels are provided inmatrix, whereby a display panel for displaying an image or the like canbe formed.

Note that in addition to the IPS mode illustrated in FIG. 11 and FIG.12, a TN (twisted nematic) mode in which a cell gap is small can beapplied. An example of a pixel of a TN mode liquid crystal displaydevice is illustrated in FIG. 13 and FIG. 14. FIG. 13 is a plan view ofthe pixel and FIG. 14 is a cross-sectional view taken along a line A-Bshown in FIG. 13. Note that FIG. 13 illustrates a plan view of thesubstrate 400 over which the transistor 610 is formed, and FIG. 14illustrates a structure in which a counter substrate 616 and a liquidcrystal layer 614 are formed in addition to a structure of the substrate600 over which a transistor 610 is formed. The following descriptionwill be given with reference to both FIG. 13 and FIG. 14.

The transistor 610 has the same structure as that of FIG. 10A, andincludes the gate electrode layer 601, the gate insulating layer 602,and the oxide semiconductor layer 603. In a pixel, the gate electrodelayer 601 is formed to extend in one direction. The oxide semiconductorlayer 603 is provided to overlap with the gate electrode layer 601 withthe gate insulating layer 602 interposed therebetween. The sourceelectrode layer 605 a and the drain electrode layer 605 b are providedover the oxide semiconductor layer 603 (note that here, the terms “thesource electrode layer 605 a” and “the drain electrode layer 605 b” areused just for convenience in order to distinguish electrodes in thetransistor 610 between source and drain). The source electrode layer 605a is extended in a direction to intersect with the gate electrode layer601. A pixel electrode 611 is provided over the protective insulatinglayer 609 which is farmed over an insulating film 607, and the pixelelectrode 611 is connected to the drain electrode layer 605 b through acontact hole 612. The pixel electrode 611 is formed from a transparentelectrode material such as indium tin oxide, zinc oxide, or tin oxide.

A storage capacitor 619 may be provided as appropriate. When the storagecapacitor 619 is provided, the storage capacitor 619 is formed includinga capacitor wiring layer 617 formed in the same layer as the gateelectrode layer 601, and a capacitor electrode layer 618. Between thecapacitor wiring layer 617 and the capacitor electrode layers 618, thegate insulating layer 602 is extended to function as a dielectric, sothat the storage capacitor 619 are formed.

The pixel electrode 611 can control the alignment of liquid crystal byfacing a counter electrode 615 on the counter substrate 616 side. Such astructure is applied in the case of the TN (twisted nematic) mode. TheTN mode is a mode of controlling alignment of liquid crystal moleculesof a liquid crystal panel.

A counter electrode 615 is provided on the counter substrate 616 side.The liquid crystal layer 614 is provided between the substrate 600 andthe counter substrate 616. An alignment film 613 is provided to be incontact with the liquid crystal layer 614. Alignment treatment for thealignment film 613 is made by an optical alignment method or a rubbingmethod. As a liquid crystal phase of the liquid crystal layer 616, anematic phase, or the like can be used.

The following components constitutes one unit: the transistor 610 inwhich the oxide semiconductor layer 603 is provided to overlap with thegate electrode layer 601 with the gate insulating layer 602 interposedtherebetween; the pixel electrode 611 which is connected to the sourceside or the drain side of the transistor 610 and drives liquid crystal;the counter electrode 615 provided to face the pixel electrode 611; andthe liquid crystal layer 614 provided between the pixel electrode 611and the counter electrode 615. A pixel can be constituted by the one ormore of units, and the pixels are provided in matrix, whereby a displaypanel for displaying an image or the like can be formed.

In such a manner, by using a transistor including an oxide semiconductorlayer having high field-effect mobility and low off-state current inthis embodiment, a liquid crystal display device with low powerconsumption can be provided.

This embodiment can be implemented by combination with structuresdescribed in the other embodiments as appropriate.

Embodiment 4

In this embodiment, examples of a transistor including an oxidesemiconductor layer and a manufacturing method thereof will be describedin detail below with reference to FIGS. 15A to 15E. The same portion asor a portion having a function similar to those in the aboveembodiments, and repetitive description is omitted. In addition,detailed description of the same portions is not repeated.

FIGS. 15A to 15E illustrate an example of a cross-sectional structure ofa transistor. A transistor 510 illustrated in FIGS. 15A to 15E is aninverted staggered thin film transistor having a bottom gate structure,which is similar to the transistor 410 illustrated in FIG. 10A.

Hereinafter, a manufacturing process of the transistor 510 over asubstrate 505 is described with reference to FIGS. 15A to 15E.

First, a conductive film is formed over the substrate 505 having aninsulating surface, and then, a gate electrode layer 511 is formedthrough a first photolithography step. Note that a resist mask may beformed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

As the substrate 505 having an insulating surface, a substrate similarto the substrate 400 described in Embodiment 4 can be used. In thisembodiment, a glass substrate is used as the substrate 505.

An insulating film serving as a base film may be provided between thesubstrate 505 and the gate electrode layer 511. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 505, and can be formed with a single-layer structure or astacked-layer structure using one or more of a silicon nitride film, asilicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

The gate electrode layer 511 can be formed to have a single-layerstructure or a stacked-layer structure using any of a metal materialsuch as molybdenum, titanium, tantalum, tungsten, aluminum, copper,neodymium, or scandium, and an alloy material which includes any ofthese as a main component.

Next, a gate insulating layer 507 is formed over the gate electrodelayer 511. The gate insulating layer 507 can be formed by a plasma CVDmethod, a sputtering method, or the like to have a single layerstructure or a stacked-layer structure using any of a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, a siliconnitride oxide layer, an aluminum oxide layer, an aluminum nitride layer,an aluminum oxynitride layer, an aluminum nitride oxide layer, and ahafnium oxide layer.

For the oxide semiconductor in this embodiment, an oxide semiconductorwhich is made to be an i-type semiconductor or a substantially i-typesemiconductor by removing an impurity is used. Such a highly purifiedoxide semiconductor is highly sensitive to an interface state andinterface charges; thus, an interface between the oxide semiconductorlayer and the gate insulating layer is important. For that reason, thegate insulating layer that is to be in contact with a highly purifiedoxide semiconductor needs to have high quality.

For example, high-density plasma CVD using microwaves (e.g., with afrequency of 2.45 GHz) is preferably adopted because an insulating layercan be dense and have high withstand voltage and high quality. Thehighly purified oxide semiconductor and the high-quality gate insulatinglayer are in close contact with each other, whereby the interface statedensity can be reduced to obtain favorable interface characteristics.

Needless to say, another formation method such as a sputtering method ora plasma CVD method can be employed as long as the method enablesformation of a high-quality insulating layer as a gate insulating layer.Further, an insulating layer whose film quality and characteristics ofthe interface between the insulating layer and an oxide semiconductorare improved by heat treatment which is performed after formation of theinsulating layer may be formed as a gate insulating layer. In any case,any insulating layer may be used as long as the insulating layer hascharacteristics of enabling a reduction in interface state density ofthe interface between the insulating layer and an oxide semiconductorand formation of a favorable interface as well as having favorable filmquality as a gate insulating layer.

In order to contain hydrogen, a hydroxyl group, and moisture in the gateinsulating layer 507 and an oxide semiconductor film 530 as little aspossible, it is preferable to perform pretreatment for formation of theoxide semiconductor film 530. As the pretreatment, the substrate 505provided with the gate electrode layer 511 or a substrate 505 over whichthe gate electrode layer 511 and the gate insulating layer 507 areformed is preheated in a preheating chamber of a sputtering apparatus,whereby an impurity such as hydrogen or moisture adsorbed on thesubstrate 505 is removed and then, evacuation is performed. As anevacuation unit provided in the preheating chamber, a cryopump ispreferable. Note that this preheating treatment can be omitted. Further,the above preheating may be performed in a similar manner, on thesubstrate 505 in a state where a source electrode layer 515 a and adrain electrode layer 515 b have been formed thereover but an insulatinglayer 516 has not been formed yet.

Next, over the gate insulating layer 507, the oxide semiconductor film530 having a thickness greater than or equal to 2 nm and less than orequal to 200 nm, preferably greater than or equal to 5 nm and less thanor equal to 30 nm is formed (see FIG. 15A).

Note that before the oxide semiconductor film 530 is formed by asputtering method, powder substances (also referred to as particles ordust) which attach on a surface of the gate insulating layer 507 arepreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without applying a voltage to a target side, an RFpower source is used for application of a voltage to a substrate side inan argon atmosphere to generate plasma in the vicinity of the substrateto modify a surface. Note that instead of an argon atmosphere, anitrogen atmosphere, a helium atmosphere, an oxygen atmosphere, or thelike may be used.

As an oxide semiconductor for the oxide semiconductor film 530, theoxide semiconductor described in Embodiment 3 can be used. Further, SiO₂may be contained in the above oxide semiconductor. In this embodiment,the oxide semiconductor film 530 is deposited by a sputtering methodwith the use of an In—Ga—Zn—O-based oxide target. A cross-sectional viewat this stage is illustrated in FIG. 15A. Alternatively, the oxidesemiconductor film 530 can be formed by a sputtering method in a raregas (typically argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere containing a rare gas (typically argon) and oxygen.

The target used for formation of the oxide semiconductor film 530 by asputtering method is, for example, an oxide target containing In₂O₃,Ga₂O₃, and ZnO at a composition ratio of 1:1:1[molar ratio], so that anIn—Ga—Zn—O film is formed. Without limitation to the material and thecomponent of the target, for example, an oxide target containing In₂O₃,Ga₂O₃, and ZnO at 1:1:2 [molar ratio] may be used.

The filling factor of the oxide target is greater than or equal to 90%and less than or equal to 100%, preferably greater than or equal to 95%and less than or equal to 99.9%. With use of the metal oxide target withhigh filling factor, a dense oxide semiconductor film can be formed.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or a hydride has been removed be usedas a sputtering gas used for forming the oxide semiconductor film 530.

The substrate is held in a deposition chamber kept under reducedpressure, and the substrate temperature is set to 100° C. to 600° C.inclusive, preferably 200° C. to 400° C. inclusive. Formation of theoxide semiconductor film is conducted with heating the substrate,whereby the concentration of impurities included in the formed oxidesemiconductor film can be reduced. In addition, damage by sputtering canbe reduced. Then, a sputtering gas from which hydrogen and moisture areremoved is introduced into the deposition chamber where remainingmoisture is being removed, and the oxide semiconductor film 530 isdeposited with use of the above target, over the substrate 505. In orderto remove remaining moisture from the deposition chamber, anadsorption-type vacuum pump such as a cryopump, an ion pump, or atitanium sublimation pump is preferably used. The evacuation unit may bea turbo pump provided with a cold trap. In the deposition chamber whichis evacuated with use of the cryopump, a hydrogen atom, a compoundincluding a hydrogen atom, such as water (H₂O), (more preferably, also acompound including a carbon atom), and the like are removed, whereby theconcentration of impurities in the oxide semiconductor film formed inthe deposition chamber can be reduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat use of a pulse direct current power source is preferable becausepowder substances (also referred to as particles or dust) generated infilm formation can be reduced and the film thickness can be uniform.

Then, through a second photolithography step, the oxide semiconductorfilm 530 is processed into an island-shaped oxide semiconductor layer. Aresist mask for forming the island-shaped oxide semiconductor layer maybe formed by an ink-jet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

In the case where a contact hole is formed in the gate insulating layer507, a step of forming the contact hole can be performed at the sametime as processing of the oxide semiconductor film 530.

Note that the etching of the oxide semiconductor film 530 may be dryetching, wet etching, or both dry etching and wet etching. As an etchantused for wet etching of the oxide semiconductor film 530, for example, amixed solution of phosphoric acid, acetic acid, and nitric acid, or thelike can be used. In addition, ITO07N (produced by KANTO CHEMICAL CO.,INC.) may be used.

Next, the oxide semiconductor layer is subjected to first heattreatment. By this first heat treatment, the oxide semiconductor layercan be dehydrated or dehydrogenated. The temperature of the first heattreatment is higher than or equal to 400° C. and lower than or equal to750° C., preferably higher than or equal to 400° C. and lower than thestrain point of the substrate. Here, the substrate is introduced into anelectric furnace which is one of heat treatment apparatuses, heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then, the oxide semiconductorlayer is not exposed to the air so that entry of water and hydrogen intothe oxide semiconductor layer is prevented; thus, an oxide semiconductorlayer 531 is obtained (see FIG. 15B).

Further, a heat treatment apparatus used in this step is not limited toan electric furnace, and a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element may be alternatively used. For example, anRTA (rapid thermal anneal) apparatus such as a GRTA (gas rapid thermalanneal) apparatus or an LRTA (lamp rapid thermal anneal) apparatus canbe used. An LRTA apparatus is an apparatus for heating an object to beprocessed by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the first heat treatment, GRTA may be performed asfollows: the substrate is transferred and put into an inert gas heatedto a high temperature as high as 650° C. to 700° C., heated for severalminutes, and taken out from the inert gas heated to the hightemperature.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the atmosphere of nitrogen ora rare gas such as helium, neon, or argon. The purity of nitrogen or therare gas such as helium, neon, or argon which is introduced into theheat treatment apparatus is preferably set to be 6 N (99.9999%) orhigher, far preferably 7 N (99.99999%) or higher (that is, the impurityconcentration is preferably 1 ppm or lower, far preferably 0.1 ppm orlower).

After the oxide semiconductor layer is heated by the first heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or ultra-dryair (having a dew point of −40° C. or lower, preferably −60° C. orlower) may be introduced into the same furnace. It is preferable thatwater, hydrogen, or the like be not contained in the oxygen gas or theN₂O gas. Alternatively, the purity of an oxygen gas or an N₂O gas whichis introduced into the heat treatment apparatus is preferably 6 N ormore, further preferably 7 N or more (i.e., the impurity concentrationof the oxygen gas or the N₂O gas is 1 ppm or lower, preferably 0.1 ppmor lower). Although oxygen which is a main component included in theoxide semiconductor has been reduced through the elimination ofimpurities by performance of dehydration treatment or dehydrogenationtreatment, oxygen is supplied by the effect of introduction of theoxygen gas or the N₂O gas in the above manner, so that the oxidesemiconductor layer is highly purified and made to be an electricallyi-type (intrinsic) semiconductor.

Alternatively, the first heat treatment of the oxide semiconductor layercan be performed on the oxide semiconductor film 530 which has not yetbeen processed into the island-shaped oxide semiconductor layer. In thatcase, the substrate is taken out from the heat apparatus after the firstheat treatment, and then a photolithography step is performed.

Note that other than the above timing, the first heat treatment may beperformed at any of the following timings as long as it is after theoxide semiconductor layer is formed. For example, the timing may beafter a source electrode layer and a drain electrode layer are formedover the oxide semiconductor layer or after an insulating layer isformed over the source electrode layer and the drain electrode layer.

Further, in the case where a contact hole is formed in the gateinsulating layer 507, the formation of the contact hole may be performedbefore or after the first heat treatment is performed on the oxidesemiconductor film 530.

Alternatively, an oxide semiconductor layer may be formed through twodeposition steps and two heat treatment steps. The thus formed oxidesemiconductor layer has a thick crystalline region (single crystallineregion), that is, a crystalline region the c-axis of which is aligned ina direction perpendicular to a surface of the layer, even when a basecomponent includes any of an oxide, a nitride, a metal, or the like. Forexample, a first oxide semiconductor film with a thickness greater thanor equal to 3 nm and less than or equal to 15 nm is deposited, and firstheat treatment is performed in a nitrogen, oxygen, rare gas, or dry airatmosphere at 450° C. to 850° C. inclusive, preferably 550° C. to 750°C. inclusive, so that the first oxide semiconductor film has acrystalline region (including a plate-like crystal) in a regionincluding its surface. Then, a second oxide semiconductor film which hasa larger thickness than the first oxide semiconductor film is formed,and second heat treatment is performed at 450° C. to 850° C. inclusiveor preferably 600° C. to 700° C. inclusive, so that crystal growthproceeds upward with use of the first oxide semiconductor film as a seedof the crystal growth and the whole second oxide semiconductor film iscrystallized. In such a manner, the oxide semiconductor layer having athick crystalline region may be obtained.

Next, a conductive film to be the source and drain electrode layers(including a wiring formed in the same layer as the source and drainelectrode layers) is formed over the gate insulating layer 507 and theoxide semiconductor layer 531. The conductive film to be the source anddrain electrode layers can be formed using the material which is usedfor the source electrode layer 405 a and the drain electrode layer 405 bdescribed in Embodiment 3.

By performance of a third photolithography step, a resist mask is formedover the conductive film, and selective etching is performed, so thatthe source electrode layer 515 a and the drain electrode layer 515 b areformed. Then, the resist mask is removed (see FIG. 15C).

Light exposure at the time of the formation of the resist mask in thethird photolithography step may be performed using ultraviolet light,KrF laser light, or ArF laser light. A channel length L of thetransistor formed later is determined by the distance between the loweredge portion of the source electrode layer and the lower edge portion ofthe drain electrode layer which are next to each other over the oxidesemiconductor layer 531. In the case where a channel length L is lessthan 25 nm, light exposure for formation of the resist mask in the thirdphotolithography step may be performed using extreme ultraviolet lighthaving an extremely short wavelength of several nanometers to severaltens of nanometers. In the light exposure by extreme ultraviolet light,the resolution is high and the focus depth is large. For these reasons,the channel length L of the transistor to be formed later can be in therange of 10 nm to 1000 nm inclusive, and the circuit can operate athigher speed.

In order to reduce the number of photomasks used in a photolithographystep and reduce the number of steps, an etching step may be performedwith the use of a multi-tone mask which is a light-exposure mask throughwhich light is transmitted to have a plurality of intensities. A resistmask formed with use of a multi-tone mask has a plurality of thicknessesand further can be changed in shape by etching; therefore, the resistmask can be used in a plurality of etching steps for processing intodifferent patterns. Therefore, a resist mask corresponding to at leasttwo or more kinds of different patterns can be formed with onemulti-tone mask. Thus, the number of light-exposure masks can be reducedand the number of corresponding photolithography steps can be alsoreduced, whereby simplification of a process can be realized.

Note that when the conductive film is etched, the optimum etchingcondition is desirably made so that the oxide semiconductor layer 531can be prevented to be etched together with the conductive film anddivided. However, it is difficult to attain such a condition that onlythe conductive film is etched and the oxide semiconductor layer 531 isnot etched at all. In etching of the conductive film, the oxidesemiconductor layer 531 is partly etched in some cases, whereby theoxide semiconductor layer having a groove portion (a depressed portion)is formed.

In this embodiment, since a titanium (Ti) film is used as the conductivefilm and the In—Ga—Zn—O-based oxide semiconductor is used for the oxidesemiconductor layer 531, an ammonium hydrogen peroxide mixture (a mixedsolution of ammonia, water, and hydrogen peroxide) is used as anetchant.

Next, by plasma treatment using a gas such as N₂O, N₂, or Ar, water orthe like adsorbed to a surface of an exposed portion of the oxidesemiconductor layer may be removed. In the case where the plasmatreatment is performed, the insulating layer 516 which serves as aprotective insulating film in contact with part of the oxidesemiconductor layer is formed without exposure to the air.

The insulating layer 516 can be formed to a thickness of at least 1 nmby a method by which an impurity such as water or hydrogen does notenter the insulating layer 516, such as a sputtering method asappropriate. When hydrogen is contained in the insulating layer 516, thehydrogen enters the oxide semiconductor layer or extracts oxygen fromthe oxide semiconductor layer, which causes a reduction in resistance ofa back channel of the oxide semiconductor layer (i.e., makes an n-typeback channel), so that a parasitic channel might be formed. Therefore,it is important for the insulating layer 516 that hydrogen is not usedin a formation method in order to contain hydrogen as little aspossible.

In this embodiment, a silicon oxide film is formed to a thickness of 200nm as the insulating layer 516 by a sputtering method. The substratetemperature in film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The silicon oxide film can be formed by a sputtering methodin a rare gas (typically argon) atmosphere, an oxygen atmosphere, or amixed atmosphere containing a rare gas and oxygen. As a target, asilicon oxide target or a silicon target may be used. For example, thesilicon oxide film can be formed using a silicon target by a sputteringmethod in an atmosphere containing oxygen. As the insulating layer 516which is formed in contact with the oxide semiconductor layer, aninorganic insulating film which does not include an impurity such asmoisture, a hydrogen ion, or OH⁻ and blocks the entry of the impurityfrom the outside is used. Typically, a silicon oxide film, a siliconoxynitride film, an aluminum oxide film, an aluminum oxynitride film, orthe like is used.

As in the case of formation of the oxide semiconductor film 530, anadsorption-type vacuum pump (such as a cryopump) is preferably used inorder to remove remaining moisture in a deposition chamber of theinsulating layer 516. When the insulating layer 516 is deposited in thedeposition chamber which is evacuated with use of a cryopump, theconcentration of an impurity contained in the insulating layer 516 canbe reduced. Alternatively, the evacuation unit used for removal of theremaining moisture in the deposition chamber may be a turbo pumpprovided with a cold trap.

It is preferable that a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, or a hydride have been removed beused as a sputtering gas used for forming the insulating layer 516.

Next, second heat treatment is performed in an inert gas atmosphere oran oxygen gas atmosphere (preferably at from 200° C. to 400° C., e.g.250° C. to 350° C. inclusive). For example, the second heat treatment isperformed in a nitrogen atmosphere at 250° C. for one hour. The secondheat treatment is performed in such a condition that part (a channelformation region) of the oxide semiconductor layer is in contact withthe insulating layer 516.

As described above, an impurity such as hydrogen, moisture, a hydroxylgroup, or a hydride (also referred to as a hydrogen compound) isintentionally removed from the oxide semiconductor film by subjectingthe oxide semiconductor layer to the first heat treatment, and thenoxygen which is one of main components of the oxide semiconductor can besupplied by the second heat treatment and has been reduced in the stepof removing impurities. Through the above steps, the oxide semiconductorlayer is highly purified and is made to be an electrically i-type(intrinsic) semiconductor. Note that the hydrogen concentration in thehighly-purified oxide semiconductor layer 304 a and the second oxidesemiconductor layer 306 a is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less. Note thatthe above hydrogen concentration of the oxide semiconductor film ismeasured by secondary ion mass spectrometry (SIMS).

Through the above process, the transistor 510 is formed (see FIG. 15D).

When a silicon oxide layer having a lot of defects is used as theinsulating layer 516, an impurity such as hydrogen, moisture, a hydroxylgroup, or a hydride contained in the oxide semiconductor layer can bediffused into the insulating layer 516 by the heat treatment which isperformed after the formation of the silicon oxide layer, so thatimpurities in the oxide semiconductor layer can be further reduced.

A protective insulating layer 506 may be formed over the insulatinglayer 516. For example, a silicon nitride film is formed by an RFsputtering method. Since an RF sputtering method has high productivity,it is a preferable method used for formation of the protectiveinsulating layer. As the protective insulating layer, an inorganicinsulating film which does not include an impurity such as moisture andblocks entry of the impurity from the outside, e.g., a silicon nitridefilm, an aluminum nitride film, or the like is used. In this embodiment,the protective insulating layer 506 is formed using a silicon nitridefilm (see FIG. 15E).

In this embodiment, as the protective insulating layer 506, a siliconnitride film is formed by heating the substrate 505 over which the stepsup to and including the formation step of the insulating layer 516 havebeen done, to a temperature of 100° C. to 400° C., introducing asputtering gas including high-purity nitrogen from which hydrogen andmoisture are removed, and using a silicon semiconductor target. In thatcase also, it is preferable that remaining moisture be removed from adeposition chamber in the formation of the protective insulating layer506 as in the case of the insulating layer 516.

After the formation of the protective insulating layer, heat treatmentmay be further performed at a temperature from 100° C. to 200° C.inclusive in the air for 1 hour to 30 hours inclusive. This heattreatment may be performed at a fixed heating temperature.Alternatively, the following change in the heating temperature may beconducted plural times repeatedly: the heating temperature is increasedfrom room temperature to a temperature of 100° C. to 200° C. inclusiveand then decreased to room temperature.

A transistor including an oxide semiconductor layer which ismanufactured in accordance with this embodiment as described achieveshigh filed-effect mobility and thus can operate at high speed. It isfound that color breaking is reduced by increase in frame frequency.When the transistor is used in a pixel portion in the liquid crystaldisplay device, color breaking can be suppressed by increase in framefrequency and a high-quality image can be provided. In addition, ahighly purified oxide semiconductor layer can be formed withoutprocesses such as laser irradiation, and enable a transistor to beformed over a large substrate, which is preferable.

This embodiment can be implemented by combination with structuresdescribed in the other embodiments as appropriate.

Embodiment 5

A liquid crystal display device disclosed in this specification can beapplied to a variety of electronic appliances (including game machines).Examples of electronic appliances are a television set (also referred toas a television or a television receiver), a monitor of a computer orthe like, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic devices each including the liquid crystal displaydevice described in the above embodiment are described.

FIG. 16A illustrates an example of the electronic book devices. Theelectronic book reader illustrated in FIG. 16A includes two housings, ahousing 1700 and a housing 1701. The housing 1700 and the housing 1701are combined with a hinge 1704 so that the electronic book reader can beopened and closed. With such a structure, the e-book reader can beoperated like a paper book.

A display portion 1702 and a display portion 1703 are incorporated inthe housing 1700 and the housing 1701, respectively. The display portion1702 and the display portion 1703 may be configured to display one imageor different images. In the case where the display portion 1702 and thedisplay portion 1703 display different images, for example, a displayportion on the right side (the display portion 1702 in FIG. 16A) candisplay text and a display portion on the left side (the display portion1703 in FIG. 16A) can display graphics.

Further, FIG. 16A illustrates an example in which the housing 1700 isprovided with an operation portion and the like. For example, thehousing 1700 is provided with a power supply input terminal 1705, anoperation key 1706, a speaker 1707, and the like. With the operation key1706, pages can be turned. Note that a keyboard, a pointing device, orthe like may be provided on the surface of the housing, on which thedisplay portion is provided. Further, an external connection terminal(an earphone terminal, a USB terminal, a terminal that can be connectedto various cables such as a USB cable, or the like), a recording mediuminsert portion, or the like may be provided on the back surface or theside surface of the housing. Further, a function of an electronicdictionary may be provided for the electronic book reader illustrated inFIG. 16A.

FIG. 16B illustrates an example of a digital photo frame including aliquid crystal display device. For example, in the digital photo frameillustrated in FIG. 16B, a display portion 1712 is incorporated in ahousing 1711. The display portion 1712 can display various images. Forexample, the display portion 1712 can display data of an image takenwith a digital camera or the like and function as a normal photo frame.

Note that the digital photo frame illustrated in FIG. 16B is providedwith an operation portion, an external connection terminal (a USBterminal, a terminal which can be connected to a variety of cables suchas a USB cable, and the like), a recording medium insertion portion, andthe like. Although these components may be provided on the surface onwhich the display portion is provided, it is preferable to provide themon the side surface or the back surface for the design of the digitalphoto frame 9700. For example, a memory storing data of an image takenwith a digital camera is inserted in the recording medium insertionportion of the digital photo frame, whereby the image data can betransferred and then displayed on the display portion 1712.

FIG. 16C illustrates an example of a television set in which a displaydevice such as a liquid crystal display device or a light-emittingdisplay device is used. In the television set illustrated in FIG. 16C, adisplay portion 1722 is incorporated in a housing 1721. The displayportion 1722 can display an image. Further, the housing 1721 issupported by a stand 1723 here. The liquid crystal display devicedescribed in any of the above embodiments can be used in the displayportion 1722.

The television set illustrated in FIG. 16C can be operated with anoperation switch of the housing 1721 or a separate remote controller.Channels and volume can be controlled by operation keys of the remotecontroller, so that images displayed on the display portion 1722 can becontrolled. Further, the remote controller may be provided with adisplay portion for displaying data output from the remote controller.

FIG. 16D illustrates an example of a mobile phone including a liquidcrystal display device. The mobile phone handset illustrated in FIG. 16Dis provided with a display portion 1732 incorporated in a housing 1731,an operation button 1733, an operation button 1737, an externalconnection port 1734, a speaker 1735, a microphone 1736, and the like.

The display portion 1732 of the mobile phone handset illustrated in FIG.16D is a touch panel. By touching the display portion 1732 with a fingeror the like, contents displayed on the display portion 1732 can becontrolled. Further, operations such as making calls and texting can beperformed by touching the display portion 1732 with a finger or thelike.

This embodiment can be implemented by combination with structuresdescribed in the other embodiments as appropriate.

This application is based on Japanese Patent Application serial No.2010-080794 filed with Japan Patent Office on Mar. 31, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A method for manufacturing a display device including apixel, wherein the pixel includes a display element and a transistor forcontrolling the display element, wherein a channel formation region ofthe transistor includes an oxide semiconductor layer, and wherein theoxide semiconductor layer is formed via a step of reducing hydrogen anda step of supplying oxygen therein.
 3. The method for manufacturing thedisplay device according to claim 2, wherein a hydrogen concentration ofthe oxide semiconductor layer is 5×10¹⁷ atoms/cm³ or less.
 4. The methodfor manufacturing the display device according to claim 2, wherein thestep of reducing hydrogen and the step of supplying oxygen are performedby heat treatments.
 5. A method for manufacturing a display deviceincluding a pixel, wherein the pixel includes a display element and atransistor for controlling the display element, wherein a channelformation region of the transistor includes an oxide semiconductorlayer, wherein the oxide semiconductor layer is formed via a step ofreducing hydrogen and a step of supplying oxygen therein, and whereinthe oxide semiconductor layer is intrinsic.
 6. The method formanufacturing the display device according to claim 5, wherein ahydrogen concentration of the oxide semiconductor layer is 5×10¹⁷atoms/cm³ or less.
 7. The method for manufacturing the display deviceaccording to claim 5, wherein the step of reducing hydrogen and the stepof supplying oxygen are performed by heat treatments.